In recent years, a steep rise in prices of fossil fuels has required to develop systems that can generate electric power directly from natural energy resources. For example, solar cells using monocrystalline, polycrystalline, or amorphous silicon (Si), solar cells formed of compounds such as GaAs and CIGS (semiconductor material consisting of copper (Cu), indium (In), gallium (Ga), and selenium (Se)), and dye-sensitized photoelectric conversion elements (Gratzel cells) have been proposed or applied to practical use.
The costs of power generation by these solar cells are, however, still higher than the supply price of electricity generated from fossil fuels. This has precluded spread of the solar cells. In addition, a substrate of heavy glass is necessarily used, which requires reinforcement work for installation. This also raises the power generation cost.
Under such circumstances, a bulk-heterojunction-type photoelectric conversion element having a photoelectric conversion layer composed of a blend of an electron donor layer (p-type semiconductor layer) and an electron acceptor layer (n-type semiconductor layer) sandwiched between a transparent electrode and a counter electrode have been proposed as a solar cell that can generate electricity at a lower cost than that in power generation using fossil fuels, and a photoelectric conversion efficiency exceeding 5% has been reported (e.g., see Non-Patent Literature 1).
Such a bulk-heterojunction-type solar cell, other than the anode and the cathode, is formed by an application method. The bulk-heterojunction-type solar cell is therefore expected to be produced readily at low cost and become a potential solution to the high-cost power generation described above. In addition, unlike other solar cells such as the above-mentioned Si solar cells, compound semiconductor solar cells, and dye-sensitized solar cells, the process of producing the bulk-heterojunction-type solar cell does not involve any step requiring a high temperature of 160° C. or more. Such a process will enable the use of plastic substrates which are inexpensive and light-weight.
A further reduction in the power generation cost, however, requires higher photoelectric conversion efficiency, and materials for higher efficiency have been developed. The methods of increasing efficiency that have been reported necessarily include treatment such as annealing, after formation of a coating film of a bulk-heterojunction-type photoelectric conversion layer, for stabilizing the phase-separated structure of the electron donor layer (p-type semiconductor layer) and the electron acceptor layer (n-type semiconductor layer) and thus have a disadvantage of low productivity. Another disadvantage is low durability due to its instable and readily variable phase-separated structure over time despite high initial photoelectric conversion efficiency.
Methods of increasing the efficiency by forming an appropriate phase-separated structure using a solvent in an application process for forming a bulk-heterojunction-type photoelectric conversion layer have also been disclosed. For example, a method using both chlorobenzene and dichlorobenzene (e.g., see Patent Literature 1), a method using a good solvent for an electron-donating conjugated compound and an electron-accepting organic semiconductor and an amide solvent having a relative dielectric constant of 33 or more (e.g., see Patent Literature 2), a method using a solvent having a boiling point of 50 to 200° C. and a solvent having a boiling point of 150 to 300° C. (e.g., see Patent Literature 3), a method using a solvent composed of a substituted alkane (e.g., see Patent Literature 4), and a method using a solvent of a halogen-free carbocyclic compound (e.g., see Patent Literature 5) are disclosed. These methods, however, have several problems on the instable phase-separated structure, in particular, instable formation of a coating film in a short drying time in order to increase the productivity.
Accordingly, for the bulk-heterojunction-type photoelectric conversion layer, a high productivity has been requested in a roll-to-roll coating process applied to a plastic substrate, and development of an efficient method for forming a stable phase-separated structure by short-time drying has been desired.